Eridanus II: A Fossil from Reionization with an Off-Center Star Cluster

Abstract

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M_V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z~6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II's cluster is offset from the center of the galaxy by a projected distance of 23 +/- 3 pc. This offset could be an indication of a small (~50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of 0.31 +0.05/-0.06 and is aligned with the orientation of Eri II within 3 +/- 6 degrees, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.

17 pages, 12 figures, 3 tables. Accepted for publication in ApJ. Structural fitting code is available at https://github.com/jsimonastro/EriII-structural-fitting

References in corpus (35)

• The NumPy array: a structure for efficient numerical computation
• The Gaia mission
• Ultralight scalars as cosmological dark matter
• galpy: A Python Library for Galactic Dynamics
• Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
• Are the Magellanic Clouds on their First Passage about the Milky Way?
• Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds
• The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies
• Does the Fornax dwarf spheroidal have a central cusp or core?
• Understanding caustic crossings in giant arcs: characteristic scales, event rates, and constraints on compact dark matter
• Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy
• Dynamical friction in constant density cores: a failure of the Chandrasekhar formula
• Farthest Neighbor: The Distant Milky Way Satellite Eridanus II
• A trio of new Local Group galaxies with extreme properties
• The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies
• The Local Group dwarf Leo T: HI on the brink of star formation
• The RR Lyrae Period - K Luminosity relation for Globular Clusters: an observational approach
• Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for $-2.4\le$ [Fe/H] $\le +0.6$, $0.25\le Y\le 0.33$, and $-0.4\le$ [$\alpha$\Fe] $\le +0.4$
• Revised and new proper motions for confirmed and candidate Milky Way dwarf galaxies
• The structural properties and star formation history of Leo T from deep LBT photometry
• The Tidal Streams of Disrupting Subhaloes in Cosmological Dark Matter Haloes
• Deadly dark matter cusps vs faint and extended star clusters: Eridanus II and Andromeda XXV
• The Dark Matter Distributions in Low-Mass Disk Galaxies. II. The Inner Density Profiles
• The K2 M67 Study: An Evolved Blue Straggler in M67 from K2 Mission Asteroseismology
• A dwarf-dwarf merger and dark matter core as a solution to the Globular Cluster problems in the Fornax dSph
• Variable stars and stellar populations in Andromeda XXV: III. A central cluster or the galaxy nucleus?
• Cusp or core? Revisiting the globular cluster timing problem in Fornax
• BVI Photometry and the Luminosity Function of the Globular Cluster M92
• Fornax globular cluster distributions: implications for the cusp-core problem
• Rediscovery of the Sixth Star Cluster in the Fornax Dwarf Spheroidal Galaxy
• Dwarfs in the Milky Way halo outer rim: first in-fall or backsplash satellites?
• Absence of HI in the Bootes Dwarf Spheroidal Galaxy
• The STREGA survey. II. Globular Cluster Palomar 12

Cited by in corpus (29)

• Gaia early DR3 systemic motions of Local Group dwarf galaxies and orbital properties with a massive Large Magellanic Cloud
• Proper Motions, Orbits, and Tidal Influences of Milky Way Dwarf Spheroidal Galaxies
• Six More Ultra-Faint Milky Way Companions Discovered in the DECam Local Volume Exploration Survey
• Soliton Oscillations and Revised Constraints from Eridanus II of Fuzzy Dark Matter
• Tucana B: A Potentially Isolated and Quenched Ultra-faint Dwarf Galaxy at D$\approx$1.4 Mpc
• Starbursts in low-mass haloes at Cosmic Dawn. I. The critical halo mass for star formation
• The star formation history of Eridanus II: on the role of SNe feedback in the quenching of ultra-faint dwarf galaxies
• Detailed chemical abundances of stars in the outskirts of the Tucana II ultra-faint dwarf galaxy
• Metallicity Distribution Functions of 13 Ultra-Faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging
• EDGE: What shapes the relationship between HI and stellar observables in faint dwarf galaxies?
• Timing the r-Process Enrichment of the Ultra-Faint Dwarf Galaxy Reticulum II
• RR Lyrae-based Distances for 39 Nearby Dwarf Galaxies Calibrated to Gaia eDR3
• Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy
• Variable stars in Local Group Galaxies -- V. The fast and early evolution of the low-mass Eridanus II dSph galaxy
• Metallicity Distribution Function of the Eridanus~II Ultra-Faint Dwarf Galaxy from Hubble Space Telescope Narrow-band Imaging
• Lyman-$\alpha$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters
• Formation of proto-globular cluster candidates in cosmological simulations of dwarf galaxies at $z>4$
• A Statistical Detection of Wide Binary Systems in the Ultra-Faint Dwarf Galaxy Reticulum II
• On the Reionization-Era Globular Cluster in Low-Mass Galaxy Eridanus II
• The spatial distribution of globular clusters in dwarf spheroidal galaxies and the timing problem
• EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf
• Harnessing the Hubble Space Telescope Archives: A Catalogue of 21,926 Interacting Galaxies
• Star cluster survival in dark matter halos: An old cluster in Eridanus II?
• The Milky Way satellite galaxy Leo T: A perturbed cored dwarf
• A new probe of dark matter-baryon interactions in compact stellar systems
• How Many Bursts Does it Take to Form a Core at the Center of a Galaxy?
• NIHAO-LG: The uniqueness of Local Group dwarf galaxies
• walter: A Tool for Predicting Resolved Stellar Population Observations with Applications to the Roman Space Telescope
• Structural parameters and possible association of the Ultra-Faint Dwarfs Pegasus III and Pisces II from deep Hubble Space Telescope photometry